Toddler boy biting

Crystal

So my almost 3 year old son loves to give hugs and will kiss me or my husband or his sister just fine no problem. But any time my MIL tries to hug him and say goodbye, he always try to bite or lick her? Like I said he doesn’t do it to anybody else (friends) not even his grandpa, my father in law. My husband says maybe it’s because he feels threatened? I don’t know. Just curious if anybody knows or has had similar experience? Any help please?